Default roles really should be like regular roles, for the most part.
This removes a number of checks that were trying to make default roles
extra special by not allowing them to be used as regular roles.

We still prevent users from creating roles in the "pg_" namespace or
from altering roles which exist in that namespace via ALTER ROLE, as
we can't preserve such changes, but otherwise the roles are very much
like regular roles.

Based on discussion with Robert and Tom.